Aug 12, 2021
Colorado Weather Forecast | Aug. 13–18, 2021
Storm chances increase while smoke decreases — for now.
Summary
- Decent storm activity near and east of the Divide on Friday, severe storms possible in northeastern Colorado. Smoke continues to clear out, with some remaining in western Colorado.
- Saturday features isolated, high-based, gusty storms across the high country, with best storm chances in the San Juans and along the Divide.
- Better moisture on Sunday causes an uptick in storm activity and coverage across the mountains, with scattered to widespread thunderstorm activity, especially in southern Colorado. Some light smoke may return to western Colorado in northwest flow.
- A pocket of drier air on Monday limits the best thunderstorm potential to southern Colorado and near the Divide. Severe storms may be possible on the eastern plains.
- Tuesday begins a pattern change to cooler, moister weather, with increased precipitation potential on Tuesday — Thursday, especially on Wednesday. Smoke could become an issue depending on how the pattern is resolved.
- Large uncertainty by next weekend, with zonal flow somewhat likely. Smoke could get very bad again by the end of next depending on the exact nature of the pattern.
Retrospective
The past five days have been rough in Colorado as smoke reached a maximum last Saturday and has been slow to clear out. Here in Golden, we’re still dealing with somewhat unhealthy amounts of particulates in the air, but we’re slowly returning to our normal levels of pollution, i.e., unhealthy levels of ground-level ozone instead of smoke.
You’ll notice a small spike today — this was some smoke ahead of another weak cold front that has pushed through. Cooler air behind the front will keep temperatures from hitting the 90s today for most of the Front Range, and is allowing some moisture recovery in eastern Colorado which has put thunderstorms in the forecast for parts of the eastern plains.
The surface-level flow behind this front should help continue to mix smoke out of the area. However, why did the leading edge of the front push additional smoke into the state? The answer is that there’s a new source of smoke: the explosive growth of a recent fire in southeastern Montana, the Richard Spring fire, which is now 165,000+ acres.
On top of that, the Dixie Fire in California is still mostly uncontained — over 500,000 acres in size — and British Columbia is now fighting over 300 fires. This will have implications on air quality next week as a large trough traverse over the state, dragging in the smoky air to our northwest with it, though conversely, some relief may come to the west and allow firefighters to get a better handle on these blazes. More on that in a bit.
Forecast Discussion — Monsoonal Pattern Friday through Tuesday
We are transitioning to a more monsoonal pattern as we get into the weekend. Here’s how the mid-atmosphere looks on Saturday:
A large center of high pressure is located over the Great Basin, and a small low is located over the border of New Mexico, Mexico, and Texas. This will set up some robust, moisture-laden low-level flow into eastern Colorado from the Gulf of Mexico, which will mainly impact weather along and east of the Divide on Friday.
On Sunday, the low over New Mexico / Mexico retrogrades far enough west to open the tap from the Gulf of California, which is our more typical source of low-level monsoonal moisture, so the rest of Colorado, particularly the San Juans, should get in on the action especially as we get into next week.
Here are some charts. In Denver, we can see that thunderstorms become fairly likely by tomorrow afternoon and may be something to contend with as we continue into the week.
For the Front Range mountains, the thunderstorm probability is considerable for the entire weekend and into next week, with the highest coverage on Sunday.
Central Colorado is a similar story.
Finally, in the San Juans, we can really see the influence of monsoonal moisture later in the weekend and into the week.
If you’re looking to get above treeline, here’s a sorted table of thunderstorm potential through the end of the weekend for some major 14er groups:
Let’s break it down day by day, including smoke coverage.
Friday
Friday’s flow of moisture from the Gulf of Mexico puts thunderstorms on the table for much of the eastern plains and terrain near the Divide, with isolated storm chances for the rest of Colorado’s high country. Due to some dynamics aloft, instability in northeast Colorado is quite high, with severe storms possible.
In fact, the latest HRRR run is sending some damaging hail through the Denver metro area, so the possibility of strong storms anywhere east of the Divide on Friday afternoon and evening should not be overlooked.
Friday still features some lingering smoke, especially in western Colorado. However, this will just be more of a general haziness than a health hazard.
Smoke is being mixed out from the east thanks to that robust low-level flow, even though we still have a decent amount of it higher in the atmosphere above Colorado. We wont really get any refills of smoke either as we progress into the weekend thanks to strong high pressure off the coast of the Pacific Northwest dragging the smoke plume from the Dixie Fire northwest into the ocean.
Saturday
A bit of a transition day, as the flow from both the Gulf of Mexico and Gulf of California is disrupted. A weak monsoonal moisture pulse should reach the San Juans, which gives them the highest likelihood of seeing thunderstorms. The Divide remains the other focal point, as there’s just enough moisture to drive thunderstorm development, though these will be of the high-based, gusty, lightning-y variety, instead of efficient rain producers.
On the Euro’s lightning product, we can see the greatest chance of seeing lightning is in the San Juans, especially the southwestern San Juans. Lightning potential looks to be the lowest in the Sangres.
Smoke continues to clear out, with perhaps a bit of subsidence keeping the Front Range urban corridor a little hazy.
Sunday
A better pulse of monsoonal moisture and some wraparound moisture from the northwest makes thunderstorms very likely across most of the state, with the Euro’s lightning product sharing the love fairly equally, though focusing again on the San Juans, with other winners being the Sangres and the Divide.
Some of the mid-level northwesterly flow (around the center of high pressure over the Great Basin) could start advecting some older dispersed smoke into the area again, which would likely be somewhat noticeable in western Colorado. However, models don’t think air quality will take a significant nosedive.
We may see some progress being made on the Dixie Fire as better moisture recovery occurs in the area.
Monday
A pocket of dry air works itself into the state by Monday afternoon, which relegates thunderstorm activity more towards the San Juans, which will still be picking up lower-level moisture from monsoonal wind patterns.
A potential dryline on the eastern plains opens the door for some severe storms, though models are in disagreement over the amount of wind shear aloft. The Euro model looks most optimistic (higher potential) in regards to the likelihood of supercells and tornadic activity.
Northwest winds aloft, due to anticyclonic flow around the high pressure to our west, continues to advect smoke towards — but perhaps not into — the state. Will denser stuff reach Colorado by the end of Monday? Some models don’t think so…but we’re not sure we share the same level of optimism due to the Euro wanting to retrograde the center of high pressure much further west than other models, which would accelerate this process — it may be on to something. Regardless, you can see some major changes just to the northwest of the state on the SILAM model, though the heavy stuff is kept at bay into at least Tuesday morning.
Tuesday
We begin a pattern change on Tuesday, with high pressure eroding quickly as a trough drops into the Pacific Northwest and starts to track east.
Ensembles still have some high pressure over the desert southwest, so we’ll probably see another push of monsoonal moisture into southwestern Colorado. By this point, we’re really starting to lose model agreement on some of the important smaller-scale features in the western United States, but at face value we would expected scattered storms in the San Juans and isolated to scattered storms throughout the rest of Colorado’s mountains, as many solutions have plenty of moisture in the area on Tuesday.
Whether or not heavy smoke really impacts Colorado is up to a few factors that haven’t been adequately resolved by models yet. If the trough over the Pacific Northwest comes in with more positive tilt, and/or the high pressure over the desert southwest is further south and stronger, we could definitely see a large amount of smoke entering Colorado.
On the spaghetti chart below for the GFS model ensemble, you can see an impactful spread between solutions, and that’s without considering the differences between this ensemble and the European or Canadian ensembles.
If the trough comes in with less positive tilt, high pressure to our southwest is less pronounced, or the trough is a bit deeper with more meridional flow, smoke may just miss us to the north. There’s not enough model agreement to lean in a particular direction, but as a hunch, we think the high pressure may end up resolving more strongly and the trough may come in a little deeper and quicker, which we think slightly favors the smokier solution.
Pattern Change: Wednesday to Next Weekend
By the middle of next week, the trough will have dropped into the western United States.
This trough looks to be fairly prolonged in nature, though it flattens out or considerably by the end of the week. Ensembles only slightly lean towards troughiness by the end of the weekend, with some deterministic models (like the Euro) bringing high pressure back into the equation.
With the trough comes cooler temperatures, higher humidities, and better chances for rain. Most of the northern Rockies (BC to Wyoming) have very little chance of picking up precipitation between now and Tuesday, but afterwards, prospects look much better:
We’ll also likely see a fairly prolonged cooldown in our neck of the woods:
Precipitation chances will reach a maximum in Colorado on Wednesday as the trough begins to traverse into the western United States, along with elevated chances on Thursday.
Importantly, flow will likely be fairly zonal over the western United States by Friday or Saturday. If the fires in California and British Columbia have not been controlled by the middle of the week (unlikely), we’re very likely to be directly downwind of the smoke plumes. We think it’s likely that air quality could be significantly impacted as we get into the end of next week, even if we managed to miss out the denser smoke before then, and we think it’s possible that much heavier smoke could enter Colorado as the axis of the trough enters the state on Wednesday. That said, if high pressure returns to the southwestern United States by the weekend, we may not have too bad of smoke concerns by then.
Otherwise, hard to say exactly what the weather will look like by next weekend — could be monsoonal in nature, or could feature zonal flow with some embedded disturbances — either way, we’re not seeing any strong guidance for heavy precipitation, but a lot could change by then.
Keep an eye on our social media, we’ll be posting updates as we get closer to the weekend.
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